The present invention relates to a hydraulic control system. More particularly this invention concerns an electro-hydraulic arrangement for connecting a source of fluid under pressure to a load.
Commonly assigned U.S. Pat. Nos. 3,768,375 and 3,744,374 describe hydraulic systems related to those of the present invention.
A hydraulic control system is known wherein the high-pressure and low-pressure sides of a source of fluid under pressure, e.g. a pump, are connectable through a three-position main valve to a pair of output lines which may, for instance, be connected to opposite chambers of a double-acting cylinder. In one end position of the valve body the one output line is connected to the high-pressure side and the other to the low-pressure side and in the opposite end position the lines are reversed. In the middle position the lines are disconnected from the source of fluid under pressure and, instead, a shunt line is connected between the high-pressure and low-pressure sides to unload the pump. Upstream of the main control valve between it and the high pressure side of the source there is connected a pressure-reducing valve whose output is connected to control valves to the low-pressure side of the source. These control valves operate opposite chambers in a double-acting cylinder that is connected to the valve body and, therefore, serve to operate the main valve. Normally the main valve is carried on a single plate and there is provided on another plate an accumulator which is pressurized through a pressure-reducing valve. The control arrangement including the above-mentioned pilot valves contains a solenoid valve which, when the system has not connected the main valve in either of its end positions, cuts the accumulator off from the source of fluid under pressure.
In such an arrangement the pump frequently operates against a considerable load. Furthermore, the third valve in the control arrangement, through which a constant small flow takes place, frequently clogs up when the oil becomes dirty so that the system fails to operate altogether. In addition, the provision of this third valve and the accumulator complicates the device to make it more failure prone and increases its cost.